This invention relates to improvements in flameholders used in conjunction with gas turbine engines having afterburners.
In such afterburners, fuel is injected into the hot gas stream generated by the engine to augment the energy level of the gas stream and thereby achieve additional thrust. The function of the flameholder is to establish a stabilized local combustion zone for the augmenting fuel.
Flameholders are generally ring-shaped and have inner and outer coaxial walls, called shrouds, which diverge from each other in the downstream direction of gas flow from the core engine. A flameholder gutter having a U-shaped (or V-shaped) cross section is positioned between the inner and outer shrouds, such that the apex, i.e., the outer surface of the U-shaped nose of the gutter, faces the upstream end of the flameholder. The flameholder is axially aligned with the exhaust opening from the core engine such that exhaust gases from the core engine, which gases includes vitiated air, are directed over the flameholder gutter. A fuel system, including a fuel spray ring, delivers a scheduled fuel supply to the zone immediately in front of the flameholder. A fuel igniter protrudes into the interior of the flameholder gutter and is held in positioned by a tubular boss which is welded to the exterior of the gutter and passes freely through a hole in the outer shroud of the flameholder.
The construction and operation of several embodiments of flameholders for gas turbine engines are disclosed in the following prior art patents, which patents are incorporated herein by reference:
U.S. Pat. No. 3,646,763, issued to J. K. Arand on Mar. 7, 1972;
U.S. Pat. No. 3,765,178, issued to R. H. Hufnagel et al on Oct. 16, 1973;
U.S. Pat. No. 3,800,527, issued to R. L. Marshall et al on Apr. 2, 1974;
U.S. Pat. No. 4,259,839, issued to G. J. Bayle Laboure et al on Apr. 7, 1981; and
U.S. Pat. No. 4,315,401, issued to G. W. Beal et al on Feb. 16, 1982.
Flameholders of the type presently in use are known to be very effective devices. However, they are also known to be adversely affected and sometimes rendered inoperative in the circumstances described below.
Because of the severe weight limitations imposed on flameholders, they are usually made of thin sheet metal material. They must also operate in areas of extremely high temperatures at which the flameholder shrouds and gutter become warped. This warping is caused by the different expansions of the parts at local temperature hot spots.
Any methods used to rigidly attach the gutter to the outer shroud in the area where the igniter holder boss passes through the outer shroud will cause unnecessary stresses and a possible cracking of the flameholder. Therefore, the igniter holder boss is usually permitted to slide freely through a circular clearance hole in the outer shroud. The result is that the gap between the gutter and the outer shroud either increases or decreases with warpage, thus affecting the fuel/air mixture in this area.
If the flameholder gutter warps away from the outer shroud near the igniter location on the flameholder ring, as is the usual direction of warpage, then a larger gap is formed therebetween and the amount of vitiated air emitted by the turbine and introduced into the gutter near the igniter will increase. This in turn produces a leaner fuel/air mixture in the vicinity of the igniter and ignition of the mixture may not occur.